Adjust a material price with a cost index

Open the contract and write down four things: which method it names, which materials are adjustable, what the base date is, and what movement has to occur before an adjustment is due. This case walks the price-index method; if your contract names the published-price method instead, the base data and the arithmetic are different and the clause will say so.

Why: An adjustment computed correctly under the wrong method is worth nothing, and it is the sort of error that survives several months before anybody checks. Getting the four terms out of the contract once, in writing, saves rereading it every month and stops each month's calculation being a fresh interpretation.

Enter the index series with a value for each period, and keep it current as the publication comes out. The values you load are the published ones for your region; the product ships the mechanism and generic vocabulary rather than anybody's published book.

Why: The series is the evidence. Loading it as data means the adjustment can be recomputed and audited a year later against the same values, instead of resting on a screenshot somebody took of a bulletin that has since been superseded.

Add an escalation line to the bill's markup stack and give it the series and the two periods: the base period the contract names, and the period the work was done in. The factor comes from what the index did between them; you do not type a percentage.

Why: Where the series has no value for one of the two periods, the bill refuses rather than guessing, and a refusal is the right answer: an interpolated index value is a number nobody published and nobody can defend. It also means an adjustment that looks complete really is complete, instead of quietly containing a line that priced at zero.

Report the adjustment with the series it used, the two periods, the factor and the base value it was applied to, so the reader can follow the arithmetic without asking you for the spreadsheet.

Why: An adjustment presented as a single figure gets queried every time. The same adjustment presented with its inputs gets checked once and accepted, because the person checking it can do the multiplication themselves and stop.

Send the statement to the other party as correspondence for the period it covers, and record the agreed figure against it. Carry the agreed amount into the period's payment application yourself, on its own line, so it can be seen and queried separately from the valuation.

Why: An adjustment buried inside a valuation total is one that gets renegotiated at settlement, because nobody can find it. On its own line, submitted with its working and agreed period by period, it is closed each month rather than accumulated into an argument about two years of steel.